Findings in our laboratory indicate that intercalating agents, and also polylysine, stimulate epsilonN-methylation of certain internal histone lysine residues in rat liver nuclei incubated in the presence of S-adenosyl-methionine, whereas, several carcinogenic agents inhibit this process to different degrees. The same agents had no effect or inhibited the activity of the isolated nuclear lysine N-methyltransferase. This pronounced effect by a relatively small change in DNA structure suggests that the polar, methylatable part of arginine-rich histone molecules, DNA and methyltransferase are situated in close proximity in the chromatin complex. Intercalating agents can therefore be used to probe chromatin structure by this new approach. In addition, polylysine was also found to stimulate this process under the same conditions, with a maximum appearing at lysine/phosphate ratio: 0.5. This was found to be due to aberrant methylation of lysine-rich histone H-1. We propose to: 1. Study the effect of pH and ionic strength and phase of the cell cycle on intercalation and stimulation of histone N-methylation in isolated nuclei and chromatin. (Activity, histone fractions involved, moiety of me-lysine.) 2. Compare the effect of arcinogens and intercalating agents on histone methylation in condensed and diffuse chromatin, and in isolated 11S chromatin particles. 3. A study on the effect of the composition of chromatin on inhibition or stimulation. 4. A study on the effect of histone content of reconstituted DNA/histone/enzyme complexes on histone N-methylation. 5. The effect of intercalating agents and carcinogens on the other histone modifications: acetylation and phosphorylation.